This invention relates generally to the combustion of wood or other combustible fibrous particles and more particularly to a burning system and method for rapid combustion of fibrous particles.
Wood burning furnaces for boilers currently in use are relatively small, rarely exceeding the production of approximately 0.25 million pounds of steam per hour. Modern fossil fueled power plants which generally exceed a production rate of 3 million pounds of steam per hour can burn natural gas, oil, or powered coal interchangeably.
For rapid combustion in air, solids must be finely divided or the combustion air temperature must be high enough to quickly volatilize a signicant amount of the solid. The ratio of the volume of air to volume of solid required to burn a given volume of solid as atmospheric pressure ranges from approximately 3500 for wood to 12,000 for coal. With a particle of 74 microns or approximately 0.003 inches diameter, which is representative of 65 percent of the coal particles used in firing a powdered coal fired furnace, there would be approximately 1.6.times.10.sup.9 particles per pound of bituminous coal, assuming spherical particles. For eastern bituminous coal approximately 10.8 pounds of air per pound of coal are required for combustion. Using a density of 84 lbs./ft..sup.3 for coal and 0.07528 lbs./ft..sup.3 for air, the number of 74 micron particles of coal in a cubic ft. of air is approximately 11 million or approximately 6400 particles per cubic inch.
Pulverizing of wood to small particle sizes requires more energy than for coal. With current methods, pulverizing wood to pass a 1/2 inch screen is the approximate minimum size that is economically feasible. A representative particle of wood passing a 1/2 inch screen is 5/16 inch.times.3/64 inch. There would be approximately 45,000 of these particles per pound of wood. Using a representative figure of 6.35 pounds of air per pound of wood, a density of 42 lbs. per cubic ft. for wood, and 0.07528 lbs. per cubic ft. of air the number of particles of wood per cubic foot of air is approximately 536 or 3 particles per cubic inch. Thus the distribution of 74 micron coal particles in the combustion air is much more favorable to rapid combustion than for wood particles passing a 1/2 inch screen.
Another approach to the comparison of wood and coal is from standpoint of ratio of particle surface area to particle weight. The time required for combustion of a particle varies inversely with the ratio of surface area to particle weight. The ratio of surface area to weight for a 74 micron spherical particle of bituminous coal is 42,300 in. .sup.2 /lb. For the 5/6.times.1/16.times.3/64 wood particles it is 3342 in..sup.2 /lb. Thus, the burning rate for the coal particle would be approximately 13 times that for the wood if the basic burning rates for coal and wood were equal. The foregoing comparisons indicate in order to burn wood particles a method and system different from that currently used for powdered coal probably will be required.
One major difference between coal and wood lies with the behavior of each on being heated. Coal will begin to coke at between 160.degree. F. and 200.degree. F. depending upon the type. Therefore, the primary air with which the coal is mixed before being ignited must be kept below 160.degree. F. to 200.degree. F. to prevent the coalparticles from agglomerating in the transit duct and burner. Wood on the other hand, can be heated to approximately 600.degree. F. without agglomerating or igniting.
The pulverization of wood is more difficult than of coal because wood is fibrous and must be shredded whereas coal is brittle and shatters on impact. At present, the cost of reducing wood to a fine powder as is done with coal is excessive. Reducing wood to particles that will pass a half inch screen is approximately the economic breaking point from standpoint of minimum particle size. The wood particles that pass a half inch screen normally consist of flat slivers that vary from 1/16 inch to 1/2 inch long, 1/64 inch to 3/64 inch thick, and 1/32 inch to 1/16 inch wide. However, there are some very fine wood particles intermixed with the slivers.
The normal practice in firing pulverized coal is to mix the particles with enough 160.degree. F. to 200.degree. F. primary air to burn the volatile matter which may constitute on the order of 16 to 40 percent of the coal on a dry basis depending upon the source of the coal. The secondary air which supports combustion of the fixed carbon particles and any remaining volatiles is preheated to approximately 600.degree. F. The system is so arranged that the secondary air sweeps in, surrounding the primary air combustion zone. The volatile matter in dry wood constitutes on the order of 80 percent of the wood, approximately two to three times that for coal and the fixed carbon is only 1/4 to 1/3 that for coal. Thus, in addition to the non-coking features of wood as compared to coal, its properties are such that methods of burning will have to be different from those for coal in order to achieve rapid and efficient combustion.
Wood or fibrous particle burning systems utilizing turbines have heretofore generally not been available due to problems related to wood burning rates and undesirable turbine blade abrasion and clogging of internal turbine surfaces.
A turbine engine is useful in driving a generator for producing electrical power. Such turbine powered generator systems may have a maximum output power of 60 megawatts and have much lower installation costs, per unit of output power, than either fossil or nuclear fuel.
In general, a turbine engine includes an air compressor, a combustion chamber for burning fuel in high pressure air, and a turbine through which high pressure combustion gases are expanded to produce work. Work in excess of that required to compress the air is used to drive an electric generator.
Typically, turbine systems employ natural gas or fuel oil, rather than a fossil fuel, such as coal, because gas or oil combustion products do not include significant ash content which, when fed through the turbine blades clog internal turbine surfaces.
Additional advantages of utilizing gas turbine powered generators over boiler powered generators is that such a plant has a high degree of mobility and no water supply is required, as is needed in conventional boiler-plant type power generating systems.
Current and future sources of natural gas and fuel oils being in increasing limited supply, power generating systems utilizing turbine engines do not provide a means of satisfying ever increasing electrical energy needs. The use of combustible fibrous particles, such as wood, or fossil fuels such as coal, as fuels for turbine engines presents a significant problem because of turbine blade abrasion and clogging as hereinabove mentioned. An additional problem associated with wood or coal fuel relates to achieving combustion of such fuels with high heat release rates as necessary to run a turbine engine.
Between the two fuels, namely, wood particles and coal, the former has greater potential as a turbine fuel than the latter, because wood has significantly less ash, produced by combustion thereof, than coal. Further, wood is a replenishable fuel as opposed to coal.
A previous attempt to use wood as a turbine fuel, as for example U.S. Pat. No. 2,735,266 to Atherton, utilizes a chamber to burn and pyrolize wood on a grate to form combustible products, which are then injected into a second chamber for complete combustion thereof before introduction into a turbine. Such a system is not applicable for generation of significant amounts of power because of the low wood burning rates inherently limited by the burning of wood on a grate or pile. Further, Atherton utilizes a combustion chamber pressurized to only two or three atmospheres, hence the volume of air available for the combustion of the wood is much larger than for a higher pressure system, and consequently heat release rates are thereby limited and a large, combustion chamber is required.
The present invention enables fibrous particles to be completely burned while suspended within a combustion chamber. As hereinbefore pointed out, wood particles can be burned at high heat release rates by injecting wood particles into air at a temperature of approximately 800.degree. to 1000.degree. F., whereupon the wood surfaces immediately undergo pyrolysis as they are being fed into a combustion chamber and the resultant mixture of charred wood particles and combustion products are burned in a combustion zone in a manner similar to natural gas or propane.
The heat release attained through burning process may in the range of 100,000 to 150,000 btu/ft.sup.3 /hr. This is important in that it enables the combustion chamber to be smaller, a smaller chamber in turn enabling less ducting, and closer proximity to the turbine thereby enabling greater thermal efficiency.